Roˆles of catalytic oxidation in control of vehicle exhaust emissions
Martyn V. Twigg
Johnson Matthey Catalysts, Royston, Herts, SG8 5HE England, United Kingdom
Available online 14 August 2006
Abstract
Catalytic oxidation was initially associated with the early development of catalysis and it subsequently became a part of many industrial
processes, so it is not surprising it was used to remove hydrocarbons and CO when it became necessary to control these emissions from cars. Later
NOx was reduced in a process involving reduction over a Pt/Rh catalyst followed by air injection in front of a Pt-based oxidation catalyst. If over-
reduction of NO to NH
3
took place, or if H
2
S was produced, it was important these undesirable species were converted to NOx and SOx in the
catalytic oxidation stage. When exhaust gas composition could be kept stoichiometric hydrocarbons, CO and NOx were simultaneously converted
over a single Pt/Rh three-way catalyst (TWC). With modern TWCs car tailpipe emissions can be exceptionally low. NO is not catalytically
dissociated to O
2
and N
2
in the presence of O
2
, it can only be reduced to N
2
. Its control from lean-burn gasoline engines involves catalytic oxidation
to NO
2
and thence nitrate that is stored and periodically reduced to N
2
by exhaust gas enrichment. This method is being modified for diesel engines.
These engines produce soot, and filtration is being introduced to remove it. The exhaust temperature of heavy-duty diesels is sufficient (250–
400 8C) for NO to be catalytically oxidised to NO
2
over an upstream platinum catalyst that smoothly oxidises soot in the filter. The exhaust gas
temperature of passenger car diesels is too low for this to take place all of the time, so trapped soot is periodically burnt in O
2
above 550 8C.
Catalytic oxidation of higher than normal amounts of hydrocarbon and CO over an upstream catalyst is used to give sufficient temperature for soot
combustion with O
2
to take place.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Catalytic oxidation; Vehiculor emissions; NOx-control; Particulate control
1. Introduction
The activity of Pt in catalytic combustion was discovered by
Humphry Davy in 1817, who found hot platinum wire became
white hot in a coal gas/air mixture. He also observed
the
catalytic oxidation of ethanol and diethylether to acetaldehyde
and acetic acid over Pt, Reactions
. Three years later his
cousin, Edmund Davy
prepared Pt black, and noted its
activity in the catalytic oxidation of ethanol.
CH
3
CH
2
OH
þ
1
2
O
2
! CH
3
CHO
þ H
2
O
(1)
CH
3
CH
2
OCH
2
CH
3
þ O
2
! 2CH
3
CHO
þ H
2
O
(2)
CH
3
CHO
þ
1
2
O
2
! CH
3
CO
2
H
(3)
Do¨bereiner extended this work, and prepared the first
supported heterogeneous catalyst, based on small pipe clay
pellets
. He studied the Pt catalysed H
2
/O
2
reaction that was
incorporated into lighters that were widely sold. At this time
Peregrine Phillips worked on oxidation of SO
2
to SO
3
for
H
2
SO
4
production, Reactions
and
, and in 1831 his patent
was published
describing the catalyst as fine Pt wires or Pt in
‘‘any finely divided state’’. When it was commercialized
SO
2
þ
1
2
O
2
! SO
3
(4)
SO
3
þ H
2
O
! H
2
SO
4
(5)
many years later the supported Pt catalyst was too readily
poisoned (especially by arsenic derived from the metal sul-
phides that were burnt to produce SO
2
at that time) and the less
poison sensitive vanadium oxide-based process was introduced
. In the meantime Michael Faraday at the Royal Institution
worked on the Pt catalysed H
2
/O
2
reaction during work on
electrolysis
. He proposed catalysis involves simultaneous
adsorption of reactants on the Pt surface, and that a clean
surface is essential. Also during electroysis experiments,
Scho¨nbein in 1838 noticed when the electricity was switched
off there was a reversed potential across the Pt electrodes
.
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This was taken up by Grove who developed the first fuel cell
. Another application of Pt catalysts was selective oxidation
of NH
3
to NO for HNO
3
production shown in Reactions
.
Kuhlmann in 1838 detailed
the oxidation of NH
3
in air over
Pt sponge at 300 8C. Later Ostwald showed optimum results
were obtained with short contact time at high temperature
,
and this led to the industrial use of Pt gauze catalysts for HNO
3
production in 1910
. This increased in importance when the
Haber-Bosch process for NH
3
was scaled-up to industrial
production just before the First World War
. When it
became apparent catalytic
4NH
3
þ 5O
2
! 4NO þ 6H
2
O
(6)
2NO
þ O
2
! N
2
O
4
(7)
N
2
O
4
þ 2H
2
O
þ O
2
! 4HNO
3
(8)
oxidation could control some exhaust gas emissions from cars
Pt-based catalysts were then used widely in laboratories and
chemical plants. It was obvious their effectiveness should be
tested as autocatalysts. A variety of base metal catalysts were
also tested, but only those containing Pt and two of its allied
metals, Rh and Pd, were successful in real-world applications.
This article briefly reviews the origins of atmospheric pollution
caused by engine exhaust emissions before detailing the ways
catalytic oxidation has been used to combat this problem.
2. Atmospheric chemistry
By the 1940s and 1950s air quality problems caused by cars
were experienced in some urban cities
, especially in
locations such as the Los Angeles’ basin where temperature
inversions trap and recycle polluted air
. Gasoline oxidation
in the engine to CO
2
and H
2
O was far from completely efficient,
Reaction
, and the exhaust contained significant amounts of
hydrocarbons and lower levels of partially combusted products
like aldehydes, ketones and carboxylic acids, together with
large amounts of CO, Reaction
. Unburned fuel, hydro-
carbons formed by pyrolysis, and various oxygenated species
are called ‘‘hydrocarbons’’ and designated HC. At high
temperature during combustion in the cylinder N
2
and O
2
react to establish the endothermic equilibrium with NO,
Reaction
. This equilibrium is frozen as the hot gases are
cooled and ejected into the exhaust manifold. The combination
of NO and any NO
2
, is referred to as NOx, and more than a
1000 ppm can be present in exhaust of a gasoline engine. The
three major primary pollutants in the exhaust gas from cars are
therefore NOx, HC and CO.
HC
þ O
2
! H
2
O
þ CO
2
(9)
HC
þ O
2
! H
2
O
þ CO
(10)
N
2
þ O
2
fi
2NO
(11)
In some American cities irritating photochemical smogs
became so frequent air quality was a major health concern. The
origin of these photochemical smogs was the primary pollutants
from cars, that were of concern in their own right, that
underwent photochemical reactions to generate a strong
oxidising irritant
shows the increase in atmospheric
oxidant levels during a day in Summer in Los Angeles during
the early 1970s; peak levels were reached during the early
afternoon. This trend followed the sunlight intensity, and it was
established ozone was the main ‘‘oxidant’’ that was produced
via the photochemical dissociation of NO
2
, Reaction
followed by the reaction of the atomic oxygen with O
2
,
Reaction
, in which ‘‘M’’ is a ‘‘third body’’ that removes
energy that would otherwise cause the dissociation of O
3
.
However, it is mainly NO that is formed in engines, and not
NO
2
, and the oxidation of NO to NO
2
, Reaction
, is a third
order reaction the rate of which depends on the square of the
very low NO
NO
2
þ hn ! NO þ O
(12)
O
þ O
2
þ M ! O
3
þ M
(13)
concentration
, as in Eq.
. The formation of NO
2
from
NO is therefore extremely slow, so direct oxidation of NO was
not the route to NO
2
. The actual oxidation of NO to NO
2
in air,
the ozone precursor, involves free radical oxidation of HC or
CO, and
2NO
þ O
2
! 2NO
2
(14)
d NO
2
d t
¼ kP
O
2
P
2
NO
(15)
M.V. Twigg / Catalysis Today 117 (2006) 407–418
408
Fig. 1. Variation of ambient atmospheric ‘‘oxidant’’ levels in a California City
during a Summer day in the 1970s. The ‘‘oxidant’’ is mainly ozone, and peaked
in early afternoons.
one of the more important series of free radical reactions
leading to it is summarised in
. Overall the process
corresponds to the oxidation of hydrocarbon in the presence of
NO to give NO
2
, an aldehyde and H
2
O. The reactive aldehyde
can undergo further reactions with NO
2
to give, for example,
peroxyacetylnitrate (PAN) accordingly to Reactions
PAN is a very strong lachrymator
, and even traces of it
cause serious eye irritation and painful breathing.
CH
3
CHO
þ OH ! CH
3
CO
þ H
2
O
(16)
CH
3
CO
þ O
2
! CH
3
COO
2
(17)
CH
3
COO
2
þ NO
2
! CH
3
COO
2
NO
2
(18)
Levels of tailpipe pollutants from American cars in the mid-
1960s were typically HC 15 g/mile; CO 90 g/mile; and NOx
6 g/mile
. Engine modifications could not alone meet the
demands of the 1970 Clean Air Act
, so as a result, catalytic
systems were introduced to control exhaust emissions.
3. Choice of catalyst types
Engine exhaust is a demanding environment, and unlike the
steady-state operation of chemical plant processes
. The
catalyst must function at low temperature, resist effects of
excursions up to 1000 8C, tolerate the presence of poisons
(especially sulphur species), and not be affected by gas flow
pulsations and mechanical vibrations. At first it was necessary
to oxidise HC and CO, and catalysts containing copper and
nickel were tested, but they were sensitive to poisoning
(initially by lead, halide and sulphur compounds), and they did
not have thermal durability
. Some important properties
of selected metals are summarised in
; the Pt-group
catalysts were very active, and much work was done with Ru,
but its oxides are volatile, and it was not possible to prepare
catalyst that did not lose Ru during use
. Even Ir oxides are
too volatile at high temperatures, so this metal could not be used
. However, especially Pt, as well as Pd and Rh met the
requirements of having the nobility to remain metallic under
most operating conditions, and not have volatile oxides that led
to metal loss; these three metals have been used in autocatalysts
since their introduction
. Of these Pt is the most noble, but
when very hot and exposed to O
2
for long periods it can sinter
through a process involving migration of oxide species. Pd
forms a more stable oxide than does Pt, and this is catalytically
active in oxidation reactions. Rh
2
O
3
is readily formed from the
metal under hot oxidising conditions, Reaction
, and this
can undergo reactions
with catalyst support compounds
such as alumina as shown in Reaction
. The main role of
rhodium is in NOx reduction, and since it is reduced rhodium
that is active, it is important this can be made available rapidly
when any oxidising conditions return to being slightly reducing,
as is illustrated in Reaction
.
2Rh
þ
3
2
O
2
! Rh
2
O
3
(19)
Rh
2
O
3
þ Al
2
O
3
! Rh
2
O
3
Al
2
O
3
(20)
Rh
2
O
3
Al
2
O
3
þ 3H
2
! 2Rh þ Al
2
O
3
þ 3H
2
O
(21)
Frequently two or more metals are used in combination in
autocatalysts. Pt/Pd was used in some of the early oxidation
catalysts, as was Pt/Rh that was also used under rich conditions
for NOx reduction. Today three-way catalysts (see below)
commonly contain Pd/Rh although Pt/Rh catalysts are still used
on some cars, and other now less common formulations
combine all three metals.
4. Early oxidation catalysts
The first cars with oxidation catalysts injected air into the
rich (excess fuel and reducing) exhaust gas to provide O
2
for
oxidation of HCs and CO. Some traditional pelleted platinum
M.V. Twigg / Catalysis Today 117 (2006) 407–418
409
Table 1
Physical and chemical properties of some selected metals and their oxides relevant to their catalytic behaviour
Metal
Atomic number
Atomic weight
Density
MP/K
Reduction potential M
n+
! M
0
(n)
Oxide stability
Platinum
78
195.08
21.45
2045
1.19 (2)
Unstable oxides
Iridium
77
192.22
22.56
2683
1.16 (3)
Moderately stable oxides
Palladium
46
106.42
12.02
1825
0.92 (2)
Stable oxides
Rhodium
45
102.91
12.41
2239
0.76 (3)
Stable oxides
Osmium
76
190.2
22.59
3327
N/A (2)
Very volatile oxides
Ruthenium
44
101.07
12.37
2583
N/A (2)
Very volatile oxides
Copper
29
63.33
8.96
1357
0.34 (2)
Stable oxides
Cobalt
27
58.93
8.90
1768
0.28 (2)
Stables oxides
Nickel
28
58.69
8.90
1726
0.30 (2)
Stable oxides
Iron
26
55.85
7.87
1808
0.44 (2)
Stable oxides
Data from
. MP: melting point.
Scheme 1.
catalyst were used in a flat radial flow-like reactor. This
configuration was not ideal because of gas by-pass, but at that
time the conversions required were not as high as today and
sufficient conversions could be achieved. However, attrition of
the pellets caused by their movement against each other under
the influence of the pulsating gas flow and vibration of the
vehicle was a major concern. An alternative catalyst structure
made use of a ceramic monolithic honeycomb that overcame
these deficiencies.
For strength reasons monolithic honeycombs had relatively
low porosity that made them unsuitable as a catalyst support
,
so a thin layer of high surface area catalytically active material
was applied to the channel walls
. This layer, typically 20–
150 mm thick, is referred to as a washcoat. The process of
applying it is called washcoating and the washcoat surface area is
typically 100 m
2
/g. The monoliths made from cordierite have
exceptionally low coefficient of thermal expansion needed to
prevent them from cracking when thermally stressed during use.
Monoliths are manufactured by extruding a mixture of clay, talc,
alumina and water with various organic additions, that is dried
and fired at high temperature when cordierite is formed
.
shows one way a ceramic monolith can be retained in a
stainless steel mantle that is welded into the exhaust system. It is
wrapped in an intumescent mat typically containing inorganic
fibres (such as rock wool), vermiculite and an organic binder.
When the converter experiences temperatures above about
310 8C the organic binder decomposes and the vermiculite
exfoliates. The force of this expansion exerts a pressure on the
monolith that keeps it firmly in place for the life of the vehicle.
also shows a metal foil-based catalyst whose stainless steel
mantle can be welded directly into the exhaust system. The
impact of fitting oxidation catalysts in the exhaust systems of cars
was very significant; there was a very large reduction in HC and
CO emissions, but there was little or no effect on the NOx
emissions.
5. Control of Nox emissions
NO is thermodynamically unstable, and it is a free radical
(enthalphy of formation DH = +89.9 kJ/mol, free energy of
formation DG = + 86.3 kJ/mol) yet under practical conditions
in the presence of O
2
catalytic dissociation does not take place
, and it can only be converted to N
2
via a reductive process.
The first approach for controlling NOx from engine exhaust
was to reduce it to N
2
over a Pt/Rh catalyst in rich exhaust gas
before air was added to permit catalytic oxidation of HC and
CO
. This arrangement, and the earlier oxidation catalyst
only system are illustrated in schematically in
. The
selectivity of the catalyst used and the conditions employed for
NOx reduction had to ensure a high degree of selectivity so as
not to reduce NOx to NH
3
or SO
2
to H
2
S. It was important any
NH
3
or H
2
S formed was minimised, and that which was formed
was reoxidised over the oxidation catalyst to more acceptable
NO and SO
2,
as shown in
. Because of this any
reduction of NO to NH
3
represented an inefficiency in overall
NOx conversion. Good overall selectivity was obtained and this
system enabled markedly lower emissions of HC, CO and NOx
to be achieved in a reliable way.
6. Modern three-way catalysts (TWCs)
The gasoline engines with the earliest catalytic emissions
control systems were fuelled via carburettors that could not
precisely control the amount of fuel that was mixed with the
intake air. Often the air/fuel ratio moved randomly either side of
M.V. Twigg / Catalysis Today 117 (2006) 407–418
410
Fig. 2. Examples of a metal-based catalyst (left) and a ceramic-based cordierite
catalyst (right). The cordierite monolith is retained in a stainless steel mantle
with an intumescent mat. Vermiculite in the mat exfoliates when heated and
permanently retains the monolith in place.
Fig. 3. Schematic arrangement of oxidation catalyst and air injection point used
initially to lower HC and CO emissions (A). The later modification (B) used air
injection after a platinum/rhodium catalyst operating under rich conditions to
reduce NOx, then HC and CO were oxidised in a second stage after air injection.
In this way all three pollutants were controlled in a two stage process.
Scheme 2.
the stoichiometric point, and it was observed a Pt/Rh catalyst
could, under appropriate conditions, simultaneously convert CO
and HC (oxidations) and reduce NOx with high efficiency
. This concept became known as a three-way catalyst (TWC),
because all three pollutants are removed from the exhaust gas
simultaneously. Application of the TWC required three elements:
1. Electronic fuel injection (EFI) so precise amounts of fuel
could be metered to provide a stoichiometric air/fuel
mixture.
2. An oxygen sensor in the exhaust to provide an electrical
signal indicating if the engine is running rich or lean.
3. A microprocessor to control a feedback-loop using oxygen
sensor signals to determine the amount of fuel to be injected
under specific conditions to maintain the exhaust gas close to
the stoichiometric point.
By the early 1980s all of the elements necessary for the
operation of TWCs were available, and this became a more
efficient means of controlling HC, CO and NOx emissions than
earlier two catalyst systems; it was also more cost effective.
Soon TWCs were universally adopted.
6.1. Oxygen sensors
Residual oxygen in the exhaust gas of a stoichiometric
gasoline engine is determined by an oxygen sensor.
illustrates some of the basic features of the original switching-
type sensor that indicated if the exhaust was lean or rich. The
stabilised zirconia thimble at operating temperature is conduct-
ing, and its surface on the reference air side has a porous Pt coating
that acts as an electrode, and a similar electrode is deposited on the
exhaust gas side. These coatings are active oxidation catalysts, so
HC and CO are oxidised by any excess O
2
. A galvanic potential is
developed across the electrodes that is related to the excess
oxygen concentration in the exhaust gas. A small electric heater
inside the zirconia thimble (not shown) heats the sensor to its
operating temperature so it can be used soon after the engine is
started. The Nernst Eq.
describes the emf developed
assuming air (P
O
2
¼ 0:21 atm) is the reference gas. For this to be
meaningful in automotive applications it is important the gas
phase oxidation reactions are
emf
¼
2:303RT
F
log
0:21
P
O
2
(22)
brought to equilibrium at the electrode surface. Today signifi-
cantly more complex wide-range sensors are available
having a flat and smaller size that are essentially a combination
of a conventional sensor and a limit current or ‘‘pump cell’’ that
are separated by a diffusion zone. A voltage is applied to the
‘‘pump cell’’ that removes or adds oxygen to the oxygen sensor
location so l = 1 condition is maintained at the oxygen sensor
via a control loop. The pump cell current then provides an
output signal directly related to the excess oxygen concentra-
tion over a broad range of oxygen partial pressures, and in
practice the l range 0.7–2.5 can be measured.
6.2. Oxygen storage components
During the development of TWC formulations redox active
Ce compounds were incorporated; under lean conditions
(oxidising) they absorb oxygen, Reaction
, and under rich
(reducing) conditions oxygen is released from them, Reaction
. These reactions are a gross simplification of what actually
happens because a wide range of non-stoichiometric oxides are
involved and formation of Ce
2
O
3
only takes place under forcing
conditions such as when OBD measurements are being made,
see Section
. A recent excellent review on the structural
chemistry of cerium oxides is available
and there are good
reviews on their roˆles in TWCs
. In this way the
composition of the exhaust gas at the catalyst surface is
buffered around the stoichiometric point, and this enhances
conversion of all three pollutants, especially NOx. Thus
reactions involved in oxygen storage make use of the two easily
accessible oxidation states Ce(III) and Ce(IV). The total
oxygen storage capacity (OSC) is directly related to the amount
of cerium oxide present, although kinetically not all of this may
be available during short engine transients for kinetic reasons.
Ce
2
O
3
þ
1
2
O
2
! 2CeO
2
(23)
2CeO
2
þ CO ! Ce
2
O
3
þ CO
2
(24)
M.V. Twigg / Catalysis Today 117 (2006) 407–418
411
Fig. 4. Basic features of an original switching oxygen sensor involving a stabilised zirconia thimble that is conducting at temperatures above 300 8C. The emf
developed across the Pt electrodes is related to P
O
2
in the exhaust gas.
Since the introduction of oxygen storage components there
has been a trend for the use of increasingly thermally stable
forms. It is possible to optimise the environment around
platinum, and if this is different from that which is optimal for
rhodium it is advantageous to physically divide the catalyst into
two (or more) layers containing well-separated different active
metal dispersions with their specific promoter packages
.
Usually Pt and Pd function best in oxidation roˆles, and they are
often located in the bottom part of a two-layer TWC. Rh in the
top layer is then exposed to all of the reductant species that
reduce NOx before the exhaust gases diffuse to the lower layer
where they are oxidised. Physical separation into layers
enhances overall catalyst performance and life by preventing
alloy formation, separating otherwise incompatible promoters,
and encouraging desired reactivity by matching catalytic
functionality by imposing appropriate diffusing conditions on
reactants.
6.3. Palladium-only TWCs
By the correct use of promoters, particularly alkaline earth
and lanthanide oxides, it was possible to modify the catalytic
properties of Pd so it can function as a TWC and catalyse
reduction of NOx as well as oxidation of CO and HC
. This
entails interplay between catalysis by Pd metal and its oxide,
the presence of which can be controlled by close contact with
cations that stabilise surface oxygen. Again separating the
catalyst coating into two layers can minimise cross-contam-
ination, and help obtain long lasting high activity. There have
been numerous studies on the mechanisms of the water gas shift
and methanol synthesis reactions over Pd
and amongst
other surface intermediates formate has been suggested.
Perhaps the alkaline promoted NOx reduction reaction with
Pd-only TWCs involves the water gas shift reaction that
produces hydrogen which efficiently reduces NOx as illustrated
in
. Here it is postulated surface formate intermediates
may be involved in converting CO to H
2
as in some copper
catalysed synthesis gas reactions
, although other
mechanisms involving reduced cerium species that abstract
oxygen from NO are also possible.
shows how well a
modern TWC performs in an engine bench evaluation test, even
after it has been harshly aged to simulate performance of the
catalyst at the end of the vehicle’s life.
6.4. Substrate types
Extruded ceramic monoliths are widely used for TWC
applications, but in some situations monoliths made by rolling
metal foils are used. For example, the use of thin foil can
provide, when appropriately coated, a catalyst with low
backpressure characteristics that can be advantageous. These
metal-based catalysts can be welded directly into the exhaust
system
. More recently there were advances in extruding
thin wall ceramic monoliths, and these have been widely used.
They have relatively low thermal mass and high geometric
surface area that facilitate fast catalyst light-off after the engine
has started. The decision about which type of substrate is used,
metallic or ceramic, depends on a balance between these
properties and the overall system cost.
6.5. TWC on-board diagnostics (OBD)
Legislation demands the functioning of TWCs is periodi-
cally interrogated during actual driving, and if performance is
lower than a predetermined level it is reported and stored in the
on-board computer
. If poor performance persists a
malfunction indicator lamp (MIL) is turned on, so the driver
can have the fault corrected. The OBD system makes use of two
oxygen sensors, one upstream and one downstream of the
catalyst. By running slightly lean for a short period the oxygen
storage component in the catalyst is converted into its fully
oxidised form, at which point the engine is run slightly rich and
the time taken for the gas exiting the catalyst to become slightly
rich, as detected by the second oxygen sensor, is a direct
measure of the oxygen storage capacity. This measurement is
related to the catalytic performance, and so it can be used as a
criterion for the OBD requirement. In practice this approach, or
a modified alternative form, works very well, and
illustrates the fundamentals of monitoring OSC using two
oxygen sensors.
6.6. Gasoline car emissions legislation
The progress made in reducing exhaust emissions from
traditional gasoline cars during the first decade following the
introduction of legislation in America can be judged from the
decrease in the amount of HC, CO and NOx emitted annually
between 1970 and 1990. Initially there was around ten million
tons of HC and seventy five million tons of CO, and some five
M.V. Twigg / Catalysis Today 117 (2006) 407–418
412
Scheme 3.
Fig. 5. Engine bench performance of an aged TWC. In the vicinity of the
stoichiometric point all three pollutants are converted to CO
2
, H
2
O and N
2
with
high efficiency.
million tons of NOx emitted each year. The amount of NOx was
significant when compared with the nitrogen ‘‘fixed’’ in the
Haber-Bosch Process as ammonia mainly for fertilizer use.
During the first two decades of catalyst fitment the total HC and
CO emissions were reduced by about 70% and some 50% for
NOx. The way the legislation was tightened over the years since
catalysts were fitted to cars to control emissions is also a
measure of progress, and recent California legislation trends are
shown in
. The improvements are such, that although the
number of cars dramatically increased, the total emissions
decreased (
), and, for example, the ‘‘alert days’’ in Los
Angeles have been effectively eliminated. In fact, the most
demanding legislation in the world today, California’s HC
SULEV limit (
) is in some cases lower than ambient air.
For HC this corresponds to a reduction of about a 2000-fold
since the mid-1960s. So, although these emissions are not zero,
they are extremely low, and the improved air quality clearly
reflects this.
7. Diesel engines emissions control
In traditional stoichiometric gasoline engines the combust-
ing mixture always contains sufficient oxygen to just combine
with the fuel. In contrast, in a diesel engine oxygen is always in
excess, since only sufficient fuel is injected into compressed hot
air in the cylinder to produce the power required at a particular
instant
. The consequence of this mode of combustion is
diesel exhaust always contains excess oxygen, and while this is
advantageous for the oxidation of HCs and CO, it makes
controlling NOx emissions extremely difficult because under
M.V. Twigg / Catalysis Today 117 (2006) 407–418
413
Fig. 6. Arrangement of two oxygen sensors upstream and downstream of a three-way catalyst for monitoring catalyst characteristics during driving. When the
catalyst is active the oxygen level oscillations are damped by the oxygen storage components in the catalyst, should deactivation take place the oscillations break
through the catalyst as illustrated by the dashed traces.
Table 2
California (CARB) Emissions Standards Post-1994
Year
Category
Emissions (g/mile, FTP test)
HC
CO
NOx
PM
1993
3.40
0.40
1994
Tier 1
3.40
0.40
2003
Tier 1
3.40
0.40
2004
TLEV
1
0.125
3.40
0.40
0.08
LEV
2
0.075
3.40
0.05
0.01
2005
LEV
1
0.075
3.40
0.40
0.08
ULEV
2
0.040
1.70
0.05
0.01
2006
ULEV
1
0.040
1.70
0.20
0.04
SULEV
2
0.010
1.0
0.02
0.01
2007
ZEV
1
0
0
0
0
ZEV
2
0
0
0
0
NB. PZEV vehicles have same emission limits as SULEV
2
with 150,000 miles
durability mandated.
a
NMHC: non-methane hydrocarbons, i.e., all hydrocarbons excluding
methane.
b
NMOG: non-methane organic gases, i.e., all hydrocarbons and reactive
oxygenated hydrocarbon species such as aldehydes, but excluding methane.
Formaldehyde limits (not shown) are legislated separately.
c
FAN MOG: fleet average NMOG reduced progressively from 1994 to 2003.
d
LEV
1
type emissions categories phasing out 2004–2007.
e
LEV
2
type emissions limits phasing in 2004 onwards.
f
LEV
2
standards have same emission limits for passenger cars and
trucks < 8500 lb gross weight.
g
SULEV
2
onwards 120,000 miles durability mandated.
Fig. 7. Decrease in Stage 1 ‘‘Alert Days’’ in Los Angeles (lower decreasing
line) compared with the number of cars on the road (upper increasing line). The
decreasing peak ozone levels are shown in the upper decreasing line. Total
emissions dramatically decreased in spite of the increased number of cars.
practical conditions NOx can only be converted to N
2
by
reduction. So far European diesel car legislative NOx
emissions requirements have been met by engine control
measures alone. But, this may not be possible in the future with
lower NOx emissions limits, so some form of lean-NOx control
will then be necessary. Because of the nature of the combustion
process some carbonaceous particulate material (PM or
‘‘soot’’), is formed by diesel engines. Over recent years
engine modifications reduced the amount of PM formed, and
reliable means of controlling the remaining PM were devised
and successfully introduced. This section is concerned with the
control of these three classes of emissions associated with
diesel engines, and each of them involve the use of oxidation
catalysts.
7.1. Hydrocarbons and carbon monoxide
Catalytic oxidation of HC and CO under the lean
conditions in a diesel exhaust should be straightforward.
However, the fuel-efficient characteristics of diesel engines
results in low exhaust gas temperature, especially during low-
speed driving. This, together with SO
2
in the exhaust gas
(derived from sulphur compounds in the fuel) that is a catalyst
poison, means achieving and maintaining good low tem-
perature catalytic performance is challenging. Pt-based
catalysts are used to oxidise CO and HC, and to achieve
the performance and durability required catalyst formulations
have the Pt in a highly dispersed form, that is well stabilised
against thermal sintering. When the engine is started the
catalyst is insufficiently warm to oxidise the hydrocarbons
initially present in the exhaust, and incorporating zeolites into
the catalyst significantly improved the performance during
the so called ‘‘cold start’’. The zeolite function by adsorbing
HC so preventing them inhibiting the active platinum sites.
This improves low temperature CO and apparent HC
oxidation performance
. At higher temperature the HC
is desorbed and oxidised over the platinum catalyst sites.
shows the effect of zeolite addition to a platinum
catalyst on HC oxidation performance. The CO oxidation
performance is also improved by incorporating zeolite into
the catalyst.
7.2. NOx control under lean conditions
Although NO is thermodynamically unstable and a free
radical, under practical lean conditions it is not possible to
achieve its catalytic dissociation to O
2
and N
2
. This is because
of the high affinity of metallic catalyst surfaces for O
2
compared to that for NO or N
2
that leads to ‘‘oxygen
poisoning’’ of the metal surface (especially with Rh that is one
of the best metals for NO dissociation). The surface becomes
covered with strongly adsorbed oxygen so preventing NO
adsorption, and a reducing species is required to remove
oxygen from the surface to allow further adsorption and
dissociation of NO
. This is what takes place smoothly on a
three-way catalyst when operating around the stoichiometric
point. In contrast under lean conditions the only easy reaction
of NO is its oxidation to NO
2
, and while this is of value in the
context of controlling diesel PM emissions and storing NOx as
nitrate (vide infra), it is not helpful in the direct conversion of
NOx into N
2
.
7.2.1. NOx-trapping
NOx-trapping involves storage of NOx as a NO
3
, phase,
Reactions
and
, during lean driving, then periodically,
when the NOx-trapping material is becoming saturated, the
exhaust gas composition is made slightly reducing for a short
period. This destabilises the NO
3
and releases the stored NOx,
as in Reaction
, which is then reduced over a Rh-containing
component in the catalyst to N
2
, Reaction
. In the
presence of CO
2
the carbonate is reformed, as in Reaction
Evidence for the presence of the NO
3
phase was been obtained
from X-ray diffraction and infrared experiments. In effect,
Reaction
is like that with a TWC operating around the
stoichiometric point. In the NOx storing and the NOx release
Reactions
and
, M represents a suitably basic element,
typically an alkaline earth, or an alkali metal cation. The
oxidation of NO to NO
2
is an equilibrium reaction with a
favourable negative heat of enthalpy, so the reaction becomes
less favoured at higher temperatures. This is illustrated in
that shows the equilibrium percentage conversion of NO to NO
2
as a function of temperature in the presence of O
2
as in the
exhaust gas of a diesel engine (curve A). At temperatures above
about 450 8C the formation of NO
2
is severely thermodyna-
mically limited, this and more importantly the stability of the
NO
3
formed limits the degree of nitrate formation at higher
temperatures. At temperatures below about 250 8C the catalytic
oxidation is kinetically limited, so these two effects combine to
form a temperature region, or window, in which NOx-trapping
is practically possible. This is also illustrated schematically in
. Higher platinum loadings can improve low temperature
performance for catalytically oxidising NO while use of
extremely stable NO
3
phases, e.g., those of alkali metals,
rather than alkaline earth nitrates, can extend the high
temperature region. The data in
were derived from
thermodynamics for the metal oxides
, but in practice
carbonates are present under operating conditions. As a result
the actual high temperature parts of the curves will be shifted to
lower temperatures. A consequence of using a very stable
M.V. Twigg / Catalysis Today 117 (2006) 407–418
414
Fig. 8. The effect of incorporating zeolite into a platinum diesel oxidation
catalyst. The control of hydrocarbon emissions at low temperature is improved
by their retention in the zeolite. At higher temperatures released HC is oxidised
over the catalyst.
nitrate is it requires high temperature during periodic reductive
regeneration. Also, for a particular cation the sulphate is
invariably
2NO
þ O
2
! 2NO
2
(25)
NO
2
þ MCO
3
! MNO
3
þ CO
2
(26)
2MNO
3
! 2MO þ 2NO þ O
2
(27)
2NO
þ 2CO ! N
2
þ 2CO
2
(28)
MO
þ CO
2
! MCO
3
(29)
2SO
2
þ O
2
! 2SO
3
(30)
SO
3
þ MCO
3
! MSO
4
þ CO
2
(31)
thermodynamically more stable than the corresponding nitrate,
and as a result sulphates decompose at higher temperatures than
do nitrates. Sulphur compounds in fuel is oxidised to SO
2
during combustion in the engine, and thence catalytically to
SO
3
that becomes stored as sulphate in a NOx-trap according to
Reactions
and
. This restricts the NOx storing capacity,
and the effects of this have to be periodically reversed by
decomposing, the sulphate at relatively high temperature;
usually in excess of 600 8C.
7.2.2. Selective catalytic reduction
The second lean NOx control method is selective catalytic
reduction (SCR) where reduction of NOx successfully
competes with the reduction of oxygen, even though the latter
is present in a large excess. This is illustrated in
where the reductant is a hydrocarbon.
Under actual diesel exhaust conditions on a car, with a Pt
oxidation catalyst only moderate NOx conversions are obtained
unless high ratios of HC to NOx are used. Then the process
becomes uneconomical because of the amount of HC
consumed. Catalysts explored for HC lean-NOx control
include those containing platinum
, copper
and
iridium
, and recently there has been considerable interest
in the behaviour of silver catalysts
. Here it appears the
nature of the support (various modified aluminas) can have a
profound effect on the catalytic performance, as can the
presence of zeolite that trap hydrocarbons within the catalyst
and effectively increase the local HC concentration.
The reactivity of HCs in lean-NOx conversion depends on
their nature the catalyst and temperature; different HCs can
behave slightly differently. At higher temperatures competitive
HC oxidation becomes increasingly important, and then most
of the HC reductant is oxidised giving little opportunity for
NOx reduction. This is a consequence of the activation enthalpy
of HC combustion being significantly higher than that for NOx
reduction, and results in a restricted temperature ‘‘window’’ in
which NOx reduction can be achieved. The maximum
conversion within this temperature ‘‘window’’ can be increased
by having more HC present, but this has an economic penalty.
Catalyst formulations containing zeolite, can provide enhanced
NOx reduction due to their ability of maintaining a high
concentration of HC in the catalyst.
A feature of many lean-NOx reduction reactions is there is
insufficient reduction capability on the surface to reduce NOx
completely to N
2
, and a significant amount of N
2
O can be
formed according to
. The relative importance of this
depends on the nature of the catalyst surface concerned, the
nature and concentration of reductant, and the temperature as
well as exhaust gas flow rates, etc. Hydrogen also participates in
lean-NOx reduction, and because hydrogen is very reactive it
reduces NOx at a relatively low temperature, so its operating
window is centred at a low temperature compared to that for
most HCs.
shows the behaviour of a range of HCs in
lean-NOx reduction in a series of laboratory experiments in
which very high NOx conversions were possible.
With an appropriate catalyst ammonia can function as a
good selective NOx reductant as shown in Reactions
and
, Pt catalysts can function very well at relatively quite low
temperatures, but vanadium-based catalysts are commonly
M.V. Twigg / Catalysis Today 117 (2006) 407–418
415
Fig. 9. Theoretical representation of NOx-trap performance while undergoing
periodic reductive regeneration for formulations containing increasingly basic
absorbants (B = Ca; C = Sr; D = Li; E = Ba; F = Na; G = Cs; H = K). The
equilibrium for the oxidation of NO to NO
2
(curve A) is ‘‘pulled’’ to the right
by the more basic components that widen the operating high temperature region.
These data are based on oxide thermodynamics but carbonates are actually
present so in practice the high temperature side of the curves are displaced to the
left.
Scheme 4.
Scheme 5.
used at temperatures typical of heavy duty diesel engine
exhaust gas. High NOx conversions are possible, but oxidation
of NH
3
affords NO at high temperatures, Reaction
, so the
apparent conversion of NO decreases as increasing amounts of
NO are formed from NH
3
. The NH
3
/SCR process over
vanadium catalyst is selective for conversion of NO to N
2
with
little formation of N
2
O, and it is interesting O
2
participates in
the overall reduction process. Ammonia SCR has been used
extensively for NOx removal from power generation and
chemical plant exhaust gases
. It may be expected ammonia
SCR will be used for NOx reduction more widely in vehicle
applications in the future.
4NO
þ 4NH
3
þ O
2
! 4N
2
þ 6H
2
O
(32)
2NO
2
þ 4NH
3
þ O
2
! 3N
2
þ 6H
2
O
(33)
NH
3
þ O
2
! NO þ H
2
O
(34)
4NH
3
þ SO
2
! 4NO þ 6H
2
O
(35)
7.3. Diesel particulate control
A characteristic of older diesel engines was ‘‘black soot’’ in
their exhausts caused by the combustion process itself in which
very small ‘‘atomised’’ droplets of fuel burning in hot
compressed air left an unburnt core of fine carbon particles
onto which other species in the exhaust gas, including HCs,
sulphur compounds, NOx and water adsorbed. Recently
tremendous advances were made in the fuelling and combus-
tion processes of modern high-speed diesel engines used in
passenger cars. This involved very high pressure pumps,
injectors with an increased number of smaller nozzles, and
multiple injections. As a result soot or particulate matter (PM),
emissions have been reduced to low levels. Nevertheless, there
are still concerns about the possible health effects of diesel PM
and there is a move to eliminate this by filtration.
A variety of ceramic and sintered metal-based filters have
been developed, and the most successful are the so-called wall-
flow filter illustrated in
. A honeycomb monolithic
structure made from porous material with alternate channels
that are plugged at both ends so exhaust gas is forced through
the channel walls. PM is too large to pass through the walls, so it
is retained in the upstream side of the filter. If too much PM
accumulates backpressure across the filter will increase and
degrade engine performance, and ultimately the engine will
cease to function. It is essential the backpressure is not allowed
to rise above a predetermined limit. The most satisfactory
means of removing trapped PM is to oxidise it to CO
2
and H
2
O.
On heavy duty diesel vehicles, such as trucks and buses, the
engine is often working at high load and the exhaust
temperature is in the range 250–400 8C. Under these conditions
it is possible to use the already present NO in the exhaust gas in
a process that continually oxidises trapped PM. An oxidation
catalyst upstream of the filter oxidises HCs and CO to CO
2
and
H
2
O, and also converts NO to NO
2
that is a very powerful
oxidant, and this continually removes PM, as shown in
in which PM is represented chemically as ‘‘CH’’. The
advantage of this system is it requires no attention, but the NO
oxidation is strongly inhibited by the presence of SO
2
, so this
technology could not be introduced until low sulphur diesel fuel
became available. Now many tens of thousands of these filter
units are in service around the world on buses, trucks, and larger
delivery vehicles
The exhaust temperatures of diesel passenger cars rarely
exceed 250 8C in town driving, so use of NO
2
to combust PM is
inappropriate except when driving at higher speeds when this
reaction, in some circumstances, can keep the filter clean.
However, the key to employing filters on diesel cars is to use
M.V. Twigg / Catalysis Today 117 (2006) 407–418
416
Fig. 10. Effect of different hydrocarbons in the reduction of NOx over a
platinum catalyst under lean conditions. A wide range of reactivities are
observed, methane (not shown) is unreactive except at high temperatures. In
each case the C/NOx ratio was 14; A = n-octane; B = methylcyclopentane;
C = toluene; D = propene, E = iso-octane. Adapted from
.
Fig. 11. A schematic representation of a ceramic wall-flow filter. The arrows
indicate the gas flow through the walls. Particulate matter is retained in the
upstream side of the filter, and this has to be removed to prevent unacceptable
pressure-drop across the filter.
Scheme 6.
‘active’ approaches to cleaning PM from the filter. These
increase exhaust gas temperatures at intervals to that at which
the soot burns. The three different system architectures for car
PM filter systems are shown schematically in
. The first
utilises a platinum oxidation catalyst in front of a filter to
control HC and CO emissions, and also to oxidise NO to NO
2
for low temperature combustion of PM in the downstream filter
when driving conditions are appropriate for this to take place.
This catalyst is also used to burn partially combusted extra fuel
injected into the engine to raise the exhaust gas temperature
high enough to promote PM combustion with O
2
(usually above
550 8C). Variations of this system are already in production in
Europe, where a base metal fuel additive is used to help lower
the temperature required to combust PM with O
2
. The second
generation has an oxidation catalyst on the filter that promotes
the rate of soot combustion at higher temperatures. The benefit
of this over the first generation is that it removes the need for a
fuel additive and a means of dispensing it periodically into the
fuel tank. The presence of platinum on the filter also removes
HC and CO during times when the filter is regenerating. The
third generation does not have a separate oxidation catalyst, but
comprises only a single catalysed filter. This has all of the
necessary oxidation catalyst functionality included in it to
oxidise HC and CO during normal driving. In addition, the
catalyst oxidises NO to NO
2
to provide some passive PM
removal when this is possible, as well as periodically oxidising
extra HCs/CO to give sufficient temperature to burn PM with
O
2
when it is necessary to clean the filter. This system is the
most thermally efficient of the three types because there is only
one substrate to heat that is close to the engine so heat losses are
minimised, and the reactions on the filter surface create heat in
the direct vicinity of the PM.
There are a significant number of first generation filter
systems on the road in Europe. Second generation technology
have began to appear, and the latest third generation technology
has just been introduced into mass production. Future
legislation standards are likely to demand PM emissions levels
that will force the use of filters on all diesel cars. Given this
progress, the diesel car will soon be ‘‘seen’’ as a much more
environmentally friendly vehicle than it was previously, and
oxidation catalysts have key roˆles in this.
7.4. Combined diesel emissions control systems
In the future several diesel emissions control systems will be
combined into a single unit to minimise space requirements,
and for cost and efficiency considerations. Examples of this
include oxidation functions in third generation particulate
filters already mentioned in the previous section, and in the
future NOx control will also be included. Already oxidation
catalyst, PM filtration and ammonia SCR for NOx control on
heavy duty diesels have been ingenuously combined in a single
compact container
, and this is illustrated schematically in
. The exhaust gas first passes through a platinum
oxidation catalyst that oxidises CO and HCs, as well as
converting NO to NO
2
that continuously oxidises PM in the
filter. The exiting NOx is then reduced to N
2
over two SCR
catalysts. The ammonia here is obtained from the decomposi-
tion of urea that is sprayed into the system as an aqueous
solution, and any adventitious ammonia is prevented from
passing into the environment by a final oxidation catalyst that
would oxidise it to NO.
8. Conclusions
Over the last three decades since the introduction of the first
oxidation catalysts on cars there has been a huge reduction in HC,
CO and NOx emissions from them, and many millions of tons of
pollutants have not been released into the atmosphere. This
significantly improved urban air quality with many associated
environmental benefits. Now new emissions control systems are
being developed for the more fuel efficient (lower CO
2
) lean-
burn engines, especially for the increasingly popular modern
high-speed diesel engine. Here catalytic oxidation is used to
control CO and HC emissions. Additionally, filter systems are
being introduced to effectively eliminate particulate emissions,
that were formerly a characteristic feature of diesel engines.
Oxidation catalysts are used to produce NO
2
for low temperature
M.V. Twigg / Catalysis Today 117 (2006) 407–418
417
Fig. 12. Three filter systems used on diesel cars. The first has an oxidation
catalyst before the filter to burn partially combusted fuel to achieve high
temperatures, and a fuel additive is used to lower the PM combustion tem-
perature. No additive is employed in the second generation system, the filter is
catalysed to accelerate PM combustion. In the third generation system all of the
required catalyst functionality is incorporated in a single filter.
Fig. 13. A compact emissions control design for heavy duty diesel vehicles that
includes oxidation catalyst, SCR ammonia NOx control, and PM filter. The first
catalyst is a platinum oxidation catalyst to remove CO/HC and oxidise NO to
NO
2
, the final annular platinum oxidation catalyst is present to remove any
adventitious ammonia that may slip from the vanadium-based SCR catalyst.
soot combustion, or for oxidising high levels of HC/CO to
achieve temperatures needed to burn trapped soot in a filter.
Catalytic oxidation of NO to NO
2
is also important in NOx
control. It is the key step in storing NOx as NO
3
in NOx-traps
and obtaining an appropriate NO/NO
2
ratio is important in
optimising the performance of NH
3
SCR. All of these
improvements in controlling automotive exhaust emissions
depends on catalysis, and catalytic oxidation has key roˆles.
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Document Outline
- Rôles of catalytic oxidation in control of vehicle exhaust emissions
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